Doug Gregor 365f0afa9f Downgrade concurrency-related function type errors in existing code ...

When in "existing" Swift code that is Swift 5.x and has not adopted
concurrency, allow mismatches in function types that only involve
ABI-neutral concurrency changes (e.g., adding `@Sendable` or removing
a global actor) by downgrading the diagnostic to a warning. This
improves the story for incremental adoption of concurrency in an
existing code base.

As part of this, generalize the facility for downgrading an error to a
warning when performing diagnostics in the constraint solver, using the
new diagnostic behavior limits rather than duplicating diagnostics.

2021-12-02 10:33:01 -08:00

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